A survey instrument using a survey, such as a total station or a digital theodolite, is conventionally performed as follows. A surveying instrument is first placed in the vicinity of a survey point (reference point) so that the main body of the surveying instrument can be located substantially_directly_above_the_survey_point_before_surveying. After that, a leveling operation by which the surveying instrument is set to be horizontal is performed by using a circular bubble tube or a lateral bubble tube, and then a centering operation to adjust a centering point is performed so that the instrument center of the surveying instrument can be located directly above the survey point while observing the survey point by use of a optical plummet or a laser plummet device. In order to perform this centering operation, the surveying instrument is required to be fairly accurately set to become horizontal during the leveling operation. However, the instrument center of the surveying instrument is caused to coincide with the survey point after the surveying instrument is set horizontally, and therefore, if the instrument center of the surveying instrument is moved, a case will occur in which the leveling of the surveying instrument becomes wrong in response to this movement, so that the leveling operation and the centering operation are repeatedly performed. Moreover, an operator is obliged to observe the survey point while peering through the optical plummet whenever the centering operation is performed.
Therefore, a surveying instrument has been proposed in which a centering operation to cause the instrument center of the surveying instrument to coincide with a survey point is simplified, and, even if a deviation exists between the instrument center thereof and the survey point, a measured value is corrected in accordance with this deviation (see Patent Document 1).
In more detail, as shown in FIG. 10 to FIG. 12, an instrument height measuring target 2100 is installed on a target 2000 used as a survey point, and the target 2100 is picked up by a first CCD 1160 and a second CCD 1170 through a reflecting prism 1300, an objective lens 1140, a first beam splitter 1130, and a second beam splitter 1150. Thereafter, a dislocation (i.e., decentering amount) x in the X direction from the reference point as an image formed by allowing the first CCD 1160 to pick up is superposedly displayed on the instrument height measuring target image formed on a reticle 1120 and on an image of the reference point indicating the center of the instrument height measuring target image, and a dislocation (i.e., decentering amount) y in the Y direction from the reference point as an image formed by allowing the second CCD 1170 to pick up is superposedly displayed thereon. The dislocations x and y are then calculated on the basis of a distance between a predetermined 0 point of the X- or Y-axis and the position of a middle point of only a cross point having an interval differing from those of cross points that cross the X- or Y-axis of each circular image among concentric circles of the instrument height measuring target image. Measured values obtained by distance measurement and by angle measurement are then corrected by the dislocations x and y of a plumb position, thus making it possible to perform an accurate survey only by performing a rough positional adjustment as a centering operation.
Japanese Published Unexamined Patent Application No. 2000-28362 (pages 4 to 6, FIG. 1 to FIG. 3.